fuse attention mask (#2111)

* fuse attention mask

* lint

* use 0 beta when no attention mask re: @Birch-san

Author: williamberman

Diff for: src/diffusers/models/cross_attention.py

@@ -185,17 +185,23 @@ def get_attention_scores(self, query, key, attention_mask=None):
             query = query.float()
             key = key.float()
 
+        if attention_mask is None:
+            baddbmm_input = torch.empty(
+                query.shape[0], query.shape[1], key.shape[1], dtype=query.dtype, device=query.device
+            )
+            beta = 0
+        else:
+            baddbmm_input = attention_mask
+            beta = 1
+
         attention_scores = torch.baddbmm(
-            torch.empty(query.shape[0], query.shape[1], key.shape[1], dtype=query.dtype, device=query.device),
+            baddbmm_input,
             query,
             key.transpose(-1, -2),
-            beta=0,
+            beta=beta,
             alpha=self.scale,
         )
 
-        if attention_mask is not None:
-            attention_scores = attention_scores + attention_mask
-
         if self.upcast_softmax:
             attention_scores = attention_scores.float()
 
@@ -228,11 +234,12 @@ def __call__(self, attn: CrossAttention, hidden_states, encoder_hidden_states=No
         attention_mask = attn.prepare_attention_mask(attention_mask, sequence_length)
 
         query = attn.to_q(hidden_states)
-        query = attn.head_to_batch_dim(query)
 
         encoder_hidden_states = encoder_hidden_states if encoder_hidden_states is not None else hidden_states
         key = attn.to_k(encoder_hidden_states)
         value = attn.to_v(encoder_hidden_states)
+
+        query = attn.head_to_batch_dim(query)
         key = attn.head_to_batch_dim(key)
         value = attn.head_to_batch_dim(value)